I wanted to write a quick not about concealer. In order to choose a concealer you would need to do the same thing as when you choose foundations. (See previous post)
As far as color you need to determine what it is you want your concealer to do. Let us take undereye circles for example:
For a more natural look you need to use something with a yellow tone for lighter skinned individual and orangey colors for darker skinned beauties. For my everyday look I would go one shade lighter than my foundation. When I want a more dramatic look I might blend out an even lighter shade underneath the eye. I would also match the finish of concealer to that of my foundation. Concealer is definitely something you should try on. Please do not be scared to try different shades to get your desired effect and shade.